I'm having a little trouble with finishing a surface integral problem. The problem statement is this:

F(x,y,z) = ix + yj + zk, where S is the hemisphere z = (a^2 - x^2 - y^2)^(1/2).

The answer is supposed to be 2pi*a^3, but I'm stuck right after switching to polar coordinates. I've attached a picture of what I've done so far.

Thanks for any help!